1. Field of the Invention
The present invention relates to a motor vehicle air bag cover for covering a folded air bag of an air bag apparatus for use in an instrument panel, a door trim, a seat back, a pad of a steering wheel, etc. More particularly, the present invention relates to an air bag cover made of synthetic resin having a skin layer on the exposed exterior or surface side of the cover and a core or base layer on the back of the surface side, and having a breakable portion which breaks upon deployment of the air bag allowing the air bag to expand and project from the air bag apparatus.
2. Description of the Related Art
An air bag cover has been designed to have a core of synthetic resin and a skin layer disposed on the front surface side of the cover (see Japanese Utility Model Publication No. Sho. 63-22262, for example).
A breakable portion has been formed in the air bag cover which breaks upon deployment of an air bag allowing the air bag to project when the air bag expands. This breakable portion has been provided by forming a concave portion in the air bag cover by a working jig using high-frequency wave, heat, or ultrasonic wave (see Japanese Patent Publication No. Hei. 2-248238, No. Hei. 4-151345, No. Hei. 6-218811).
When a concave portion is provided on the front surface side of the air bag cover, the concave portion detracts from the design of the air bag cover. Thus, the concave portion has been provided on the back surface side. However, it has nonetheless been necessary to provide such a concave portion in the skin layer in order to allow the air bag cover to break smoothly.
However, the thickness of the skin layer of these known covers is so thin (about 1 mm) that it is difficult to control the thickness of the skin layer when the concave portion is formed. The thickness of the skin layer is particularly difficult to control when the air bag cover is curved and the breakable portion must be formed in the curved surface. It has been therefore very difficult to control the thickness of the skin layer.